Charged particle beam apparatuses are used in a plurality of industrial fields including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices, and testing systems. There is a high demand for structuring, testing, and inspecting specimens within the micrometer and nanometer scale. Micrometer and nanometer scale process control, inspection, or structuring is often done with charged particle beams, such as electron beams. Charged particle beams offer superior spatial resolution compared to, for example, photon beams due to their short wavelengths.
Besides resolution, throughput is an issue for such devices. Since large substrate areas may have to be patterned or inspected, a throughput larger than 10 cm2/min, for example, may be desired. In charged particle beam devices, the throughput depends on the charged particle beam current. Thus, there is a need for increasing the beam current. Generally, for the following discussion, there is no need for distinguishing between secondary electrons, back scattered electrons, and Auger electrons. Therefore, the three types together, for simplicity, will be referred to as “secondary electrons.”
In view of the desire for increased charged particle beam currents, charged particle beam emitters, such as field emission emitters, have an enormous potential due to their high brightness. Further, these emitters have a small source size and low energy spread. A (cold) field emitter typically includes a crystal of tungsten formed to a very narrow point which is mounted to a loop of tungsten wire. The very narrow point is also frequently referred to as an emitter tip. When applying a voltage to the cold field emitter, a very strong electric field is formed at the emitter tip due to its small radius of curvature. The strong electric field enables the electrons to pass the potential barrier between the metal and the vacuum in which the cold field emitter is placed. Accordingly, the established electric field is often referred to as an electric extractor field because it causes the electrons to be “extracted” from the emitter tip.
Generally, crystalline field emitters have different emission areas corresponding to different crystal surfaces or orientations on the small tip. The beam current and the emission stability of a field emission gun can depend strongly on the emission area of the field emitter. FIG. 1 shows a typical emission pattern of a [110] oriented single crystalline tungsten emitter obtained by using a field electron microscope (FEM). The brightness of different emission areas varies significantly because of the different work functions for the different crystal surfaces. In addition, the stability of the emission current differs depending on the emission corresponding to the crystal orientation. To enhance the brightness of a field emitter an additional coverage may be applied in order to reduce the work function of certain crystal surfaces, for example W(100)-ZnO Schottky emitter.
Prior art devices tend to determine the emission area by a rough mechanical alignment of the position and the crystal orientation.
In practice it is desired to improve both the brightness and the stability of the charged particle beam current, especially in the case of cold field emission.